The present invention generally relates to the production of woven glass fabrics and particularly to the drying of woven glass fabrics for use as a substrate for printed circuit boards.
Laminated circuit boards are used widely in sensitive electronic equipment such as computers and communications equipment. Circuit boards, generally, are multi-layer composites of thermosetting polymers and suitable reinforcing materials. One suitable reinforcing material is one or more layers woven glass fabric.
Such reinforcing fabrics are extremely fine, lightweight and very fragile. During processing, the fabric is particularly susceptible to picking up residue or particulate matter that, if conductive and not removed, may cause performance malfunctions (such as short circuits) in the finished printed circuit boards. Due to such sensitivity, high standards of cleanliness and quality are imposed. Fabrics commonly are rejected if the fabrics are contaminated with any foreign particles, whether conductive or not.
Heretofore, during finishing, and often the application of the selected finishing fluid, such fabrics have been dried using hot air, contact heat, or infrared radiation. Such drying techniques are acceptable for ordinary applications in which deposits of some foreign particulate matter will not adversely affect the product or may be washed out after subsequent operations. However, these techniques have proved to be unsatisfactory where the fabric is used in the manufacture of printed circuit boards. Hot air, contact heat, and/or infrared radiation techniques are prone to leave too many fine particles that may contaminate the woven glass fabric to an unacceptable level. Hot air introduces a foreign fluid stream that is riddled with particulate matter. Contact heat can cause migration from one face of the fabric to another and results in uneven and non-uniform drying. Infrared radiation cannot be controlled sufficiently; therefore a mechanical shutter system must be used to prevent excessive heating of the fabric. The shutter system introduces rust, dirt, and other particulate matter to the fabric environment. Further, some fiberglass fabric finishes are heat sensitive and are often damaged during these harsh drying processes.
Drying of fiberglass fabric used as substrates in printed circuit boards by radio frequency waves in the range of 40 MHz has been suggested, but this itself raises certain problems. First, electrodes must be spaced or xe2x80x9ctunedxe2x80x9d for each application. Secondly, the use of spaced electrodes can cause sporadic arcing which results in holes being burned in the fabric and shuts down the production line. There is uneven drying of the fabric with the edges drying last. Also, the differences between the electrical loss tangent for various finishes in radio frequency is significant and can adversely affect the drying rate.
The present invention is a process for the manufacture of woven glass fabric to be used in sensitive electronic equipment. The process adopts a drying technique in which the woven glass fabric is subjected to electromagnetic radiation in the microwave (900 MHz and above) range. Preferably the electromagnetic field is formed in a serpentine slotted waveguide.
Microwave frequency drying involves the heating of electrically non-conducting materials by a rapidly varying electromagnetic field. With respect to drying systems for sensitive fabrics, such as fiberglass fabrics for printed circuit board laminates, the use of microwave energy is believed to be previously unknown. Generally, microwave drying processes are generally not as cost-effective as other drying processes such as hot air, contact heat or infrared radiation. However, microwave drying is substantially cleaner and produces a higher quality of such specialty fabric as are used in substrates for electronic equipment. With microwave drying there is no pollution from gas or oil fired burners or electric heating elements. The reduced air volume that is required with microwave drying reduces air-based contaminants.
While RF drying achieves some of the desired results there are still some problems. As opposed to RF drying, microwave systems do not employ spaced electrodes. This eliminates any arcing through the fabric and the attendant problems. Furthermore, the drying is more uniform, drying efficiency is enhanced, and the equipment is less expensive. The electrical loss tangent for various finishes is more closely matched in the microwave region than in the RF region which means more compatible coupling with different finishes is realized.
It is an object of the present invention to more efficiently and effectively produce an improved woven glass fabric well suited for use as a reinforcing laminate for electronic applications. As in the case of RF dryers, the use of a microwave dryer eliminates the need for accumulators in the process. Accumulators are used in conventional fabric drying operations to facilitate continuous running and avoid stopping the dryer. Neither entrance nor exit accumulators are necessary with RF or microwave dryers. Accumulators are a chronic source of fabric distortion due to alignment difficulties. It is easier to handle fragile, lightweight fabrics when the accumulators are removed from the process. Moreover, the associated problems of accumulators, such as alignment, are eliminated. The dryer can be quickly stopped without accumulators to avoid damage to the glass woven fabric.